You need to define a "core" module in your drush-makefile, especially if you use custom distribution:
core = 7.x
api = 2
projects[l10n_install][type] = core
projects[l10n_install][version] = 1.0
If you do not specifiy projects[...][type] = core
, then drush make uses the most recent default drupal distribution.
You can use the option "-v" or "-d" to debug the running drush command.
Here ist a good introduction into drush-make: http://www.lullabot.com/blog/articles/drush-make-and-pressflow
Please provide the drush-make file, if you need more assistance.